The invention relates to optical correlation systems for obtaining the correlation function of one image by another. Such systems allow, for example, a predetermined graphic symbol to be recognized in a composite pattern.
One known correlation method consists in recording on a photosensitive medium a system of interference fringes representing the diffraction figure provided by a lens which corresponds to two light beams in the path of which are placed respectively two objects with non-uniform transparency, generally the object to be analyzed and a reference object. This photosensitive medium is read by a reading beam and there is obtained, in the focal plane of a second lens, an intensity distribution characteristic in certain zones of the product of correlation between the two objects. In the case where the reference object bears a pattern which we seek to find again in the object to be analyzed, the object obtained is formed of peaks indicating the presence and the position of the reference pattern in the objects to be analyzed. This method of correlation has been tested with interference-fringe support media of photographic and thermoplastic types.
Such media require a chemical or heat treatment between the recording and reading phases, which involves a time lag between the two operations. Furthermore, they are generally not erasable. So they do not allow real-time operation.